It is often desirable to obtain measurements of selected characteristics of sheet materials during manufacture. Although various properties of sheet materials can be detected by off-line laboratory testing, this procedure is often not practical because of the time required for sample acquisition and analysis. Also, laboratory testing has the shortcoming that samples obtained for testing may not accurately represent sheet material that has been produced.
To overcome the drawbacks of laboratory testing of sheet materials, various sensor systems have been developed for detecting sheet properties “on-line,” i.e., on a sheet-making machine while it is operating. Sensors for continuous flat sheet production processes typically employ single or double-sided packages with on-line sensors that traverse or scan traveling webs of sheet material during manufacture. Scanning usually is done in the cross direction, i.e., in the direction perpendicular to the direction of sheet travel. Depending upon the sheet-making operation, cross-directional distances can range up to about twelve meters or more. In the paper making art, for instance, the on-line sensors detect variables such as basis weight, moisture content, and caliper of sheets during manufacture.
On-line measurements during the production of continuous sheet materials are difficult to make accurately. One factor affecting the accuracy of on-line measurements is the fluctuation in the ambient temperature at the vicinities of the various sensors employed in the scanner system. The temperature variations may be caused by changes in the temperature of the sheet material itself as well as by external influences.